Method and device for the non-cutting production of an outside thread on hollow metal work pieces

ABSTRACT

A method for the non-cutting production of an outer thread on hollow metal work pieces for use as hollow screws, hollow shafts, threaded sleeves or fittings as stable connecting or fastening elements. The method is particularly suited for thin-walled work pieces and a device is provided for carrying out the method. Within a multistage press formed with a displaceable carriage and a stationary work piece carrier unit, a wire or hollow tube section forming the base material is reshaped in one or more reshaping stages I to V into a finished hollow blank. In the multistage press, the pre-fabricated hollow blank is inserted in a further stage VI into a multipart split tool with a cavity having an inside wall that is provided with a negative of the thread to be formed. In the closed state of the split tool, at least one mandrel is inserted into the central opening of the blank and the blank is widened, wherein the contours of the threaded profile is filled by a radial material flow and subsequently the finished work piece is removed.

The invention relates to a method for the non-cutting production of an outside thread on hollow metal work pieces for use as hollow screws, hollow shafts, threaded sleeves or fittings as stable connecting or fastening elements, and to a device suited for carrying out the method.

Two classical methods are known for producing outside threads on metal work pieces: milling or rolling or cutting of threads. These methods are used in practice dependent on the size of the thread, the shape of the work piece and/or the hardness of the metal work piece.

Machines with two or three driven round rolls or flat thread-rolling dies that move towards each other (parallel jaws) are used in milling or rolling. The thread can also be produced by thread-rolling heads clamped in lathes by means of which the work piece is driven.

The cold rolling of threads on thin-walled hollow parts by means of a profile rolling machine is known for example from DE 195 36 817 A1.

The mass production of outside threads milled or rolled on hollow cylindrical metal work pieces has become extremely important in practice. A prerequisite for this process is, however, that the hollow cylindrical work pieces should have a sufficient wall thickness. The wall thickness should be at least approximately ⅕ of the outside diameter/wall thickness for thread rolling on two roll machines and approximately ⅛ of the outside diameter/wall thickness for thread rolling on three roll machines.

The thread can be produced only by a cutting process or by means of thread-rolling heads on lathes for ratios below this value. The production of threads on multi-spindle automatic lathes or on thread-cutting machines is, however, costlier in terms of material and time than metal forming production.

It is not possible to achieve a mill-finish, strain-hardened surface where outside threads are produced by metal-cutting shaping on automatic lathes by means of cutting tools.

It is the object of the invention to devise a method for the non-cutting production of an outside thread on hollow metal work pieces for use as hollow screws, hollow shafts, threaded sleeves or fittings as stable connecting or fastening elements that is particularly suitable for thin-walled work pieces and is characterised by improved economy. In addition, a device suited for carrying out the method is to be provided.

The objective is achieved according to the invention by means of the features specified in claim 1. Advantageous developments of the method are the subject matter of claims 2 to 11. A device suited for carrying out the method is the subject matter of claim 12. Further embodiments of the device are specified in claims 13 to 21.

According to the proposed method, metal wire cut to length or bar stock is used as the base material. The wire section is reshaped in stages to a finished hollow blank within a multistage press with a displaceable carriage and a stationary work piece carrier unit in one or more extrusion or upsetting stages. In a further stage the prefabricated hollow blank is inserted within the multistage press into a multipart split tool having a cavity, the inside wall of which is provided with a negative threaded profile. To form the outer thread at least one mandrel is inserted or introduced on one side of the hollow work piece at its central opening, with the work piece being expanded by the mandrel. The contours of the threaded profile are filled via the expanding process by a radial material flow (lateral extrusion), thereby producing the outer thread.

Finally, the finished work piece is removed from the tool after the tool has been opened.

The method proposed enables outside threads to be produced on different hollow work pieces in an extremely economical manner. The metal work pieces suitable for this method are work pieces made of cold formable materials such as copper, aluminium or steel.

The work pieces produced with outside threads are intended for use as hollow screws, hollow shafts, threaded sleeves or fittings. These are used as stable connecting or fastening elements.

If necessary, the inserted blank is secured to prevent it being expanded longitudinally during the movement of the mandrel.

The opening and closing movement of the jaws of the split tool as well as the movement of the mandrel can be triggered by the central drive unit of the multistage press. Separate drive units can of course also be used though this arrangement is associated with higher expenditure.

The hollow work pieces can have a circular, oval or trilobular cross-sectional shape internally or externally as well as internally and externally (especially in the case of screws). The area of the work piece on which an outside thread is to be produced can also have a cavity subdivided into several hollow sections by a partition or other means of limitation. In this case a mandrel comprising a plurality of mandrel sections is inserted into the cavity. The individual mandrel sections are adjusted as regards their geometry to the respective hollow sections and at the same time shape the corresponding thread sections during the expanding process. The outside thread produced can have longitudinally interrupted threads which are located for example at the points where the junctions lie between the partition and the outer wall.

One intended application of this is for example oil drain plugs with two axial channels for draining the oil.

According to a preferred variant of the method, the risk of material getting into the closing gap between the jaws of the split tool during the thread pressing can be prevented by reducing the profile height of the negative threaded profile in the area of the closing edge of the jaws. This causes the formation of a reduced threaded profile at the points on the blank that are in the area of the closing edges of the jaws.

The thread is lowered or practically no longer exists in the area of the closing edges of the jaws.

Alternatively, recesses or other blanks running in an axial direction can be produced within the multistage press in one of the upstream process stages on the section on which the thread is to be formed. The recesses should then be located radially at the points where the closing edges of the jaws of the split tool lie.

The blank can also be constructed in such a way that the partition wall surrounding the cavity on which the thread is to be produced is interrupted at one or more points. This process is carried out within one of the reshaping stages before the actual thread pressing.

It is advantageous that the method is also suitable for shaping outside threads on thin-walled hollow work pieces which can be produced according to the known methods only at great expense. The minimum wall thickness of the hollow sections depends inter alia on the depth of the thread flanks of the thread to be formed and the material from which the blank is made.

A further advantage is that the thread section is strain-hardened by the expanding process and at the same time a bright surface is formed, eliminating the need for further reworking. The additional strain hardening makes it possible for the thread section to bear a higher mechanical load.

The expenditure required for technical equipment for producing the outside threads is relatively small.

The thread to be formed can also consist of sections of different thread geometry. The threaded profile of the inner wall of the cavity is then formed accordingly.

The threaded profiles can also be combined with other profiles. The outside thread produced can for example be interrupted by recesses or circular profiles, singly or in combination. Other profiles can also be produced outside the thread above and/or below. Additional profiles matched to the intended use of the respective shaped parts can therefore be produced apart from the outside threads.

With the exception of the threads, no further or additional profiles can be produced economically on the work piece or blank in one process stage according to the methods known per se for producing outside threads by milling or rolling.

The mandrel can be introduced into the cavity of the work piece or blank by an axial or rotary movement or a combined axial and rotary movement in order to produce the required extrusion force for the required radial material flow. Alternatively, the work piece can be also moved over the mandrel or both can be moved relative to each other. The mandrel can be made of high speed steel or hard metal.

The expanding process for shaping the threads can be carried out in stages with mandrels of different outer circumferences or mandrel sections.

Several mandrels of different dimensions can also be used in succession instead of one mandrel. This may prove to be necessary especially with relatively thick-walled sections. The mandrel or mandrels can have form shaping zones of different geometry. An initial section is so constructed that it has a polygonal contact surface, preferably triangular or quadrangular, in which the corners are slightly rounded. Initially, the inner wall of the work piece is expanded during the expanding process in an axial and radial direction only at the points or areas at which the corners of the mandrel are in contact with the inner wall of the blank. Accordingly, it is only at these points that material of the work piece is pressed into the threaded profile. In the following additional expanding process the outside thread is produced in its final form by means of the mandrel section which is adapted to the cross-sectional shape of the cavity. These measures counteract an axial lengthening of the work piece because the points on the threaded profile that have already been filled and strain-hardened during the first expanding step inhibit an axial lengthening during the second expanding step or make it possible only to a limited extent.

If necessary, the work piece can be heated to shaping temperature before or after insertion or feeding into the tool. The temperature depends on the wall thickness and the material from the work piece is made.

The expanding process takes place preferably in a multipart split tool with a plurality of movable jaws, for example two, three or four jaws. In shaping threads with a relatively large thread depth, considerable forces can be required for expanding the work piece. The possibility cannot therefore be excluded that the jaws of the split tool are pushed apart slightly at their contact points during the expanding process resulting in the formation of a longitudinal edge at these points. In order to counteract this, small recesses running longitudinally are made before the thread pressing process at the points where the jaws of the split tool meet in the area or section of the hollow work piece that is intended for shaping the thread.

This prevents material getting between the jaws when the jaws are opened slightly during the expanding process.

The device suited for carrying out the method comprises a multistage press with a displaceable carriage and a stationary work piece carrier unit wherein at least one multipart split tool movable into an opening and closing position is arranged within the multistage press either on the carriage or the stationary work piece carrier unit which has a cavity with at least one central opening into which a hollow blank can be inserted.

The inner wall limiting the cavity is fitted with a negative threaded profile. In addition, the device comprises at least one stop to secure the position of the work piece and at least one expanding mandrel or punch that can be inserted into the hollow work piece as well as an ejector system. The punch and/or tool or the units on which these are fitted are movable by means of a drive unit. The work piece inserted into the cavity is released for unloading by the opening movement of the tool after the work piece has been formed.

The cavity limited by the inner wall of the pressing jaws should be so dimensioned that there is only a small amount of play or contact in the closed state of the jaws between the surface area of the work piece that can be inserted and the inner wall of the cavity.

The threaded profile of the jaws can be interrupted by other profiles in which there are no threads. Such other profiles can also be arranged outside the threaded profile.

At least one stop for example is provided to secure the position of the blank during the expanding process within the multistage press.

The negative threaded profile of the jaws is preferably so constructed that the outside thread diameter on both sides continuously diminishes in the area of the peripheral zones of the jaws, i.e. the depth of profile diminishes. This significantly reduces the risk of material getting between the jaws during the thread pressing process.

The cavity limited by the inner wall of the jaws is of such dimensions that there is only a small amount of play or contact in the closed state of the jaws between the surface area of the blank that can be inserted and the inner wall of the cavity.

The threaded profile of the jaws can be interrupted by other profiles or the jaws outside the thread profile can be fitted with other profiles.

The movement of the expanding mandrel or split tool as well as the opening and closing movement of the jaws can be controlled by the movement of the carriage. A separate drive can also be provided to move the jaws.

The movement of the carriage causes a relative movement between the split tool and the mandrel. The jaws can be moved by a spring assembly that can be tensioned or released by the movement of the carriage or by a separate drive.

According to a further embodiment the expanding mandrel can also have different form shaping sections with the first section that can be introduced into the work piece being polygonal in shape with rounded off corners or edges that come into contact with the work piece.

The expanding mandrel can also comprise a plurality of longitudinally divided sections or segments between which a free space is arranged in a radial direction. The split tool comprises a single or multipart external ring in which the pressing jaws are mounted so as to be radially displaceable with the surface areas of the pressing jaws and the inner surface of the ring being of conical shape. The external ring is in contact with a movable circular component that can be moved by the carriage.

The expanding mandrel can also have different form shaping sections in which the first section that can be introduced into the work piece is polygonal in shape. The corners or edges coming into contact with the work piece have been rounded off.

The invention is now to be explained by an exemplary embodiment. The associated drawing shows the following:

FIG. 1 a two-part split tool in simplified perspective representation in longitudinal section,

FIG. 2 a pressing jaw of the split tool according to FIG. 1,

FIG. 3 a split tool with three pressing jaws in the closed state in simplified perspective representation in longitudinal section,

FIG. 4 the split tool according to FIG. 3 during the expanding process,

FIG. 5 the split tool according to FIG. 3 in the opened condition,

FIG. 6 a section along the line A-A in FIG. 3,

FIG. 7 a second embodiment of an expanding mandrel in perspective representation,

FIG. 8 a section along the line B-B in FIG. 7,

FIG. 9 a third embodiment of an expanding mandrel in perspective representation and

FIG. 10 a section along the line C-C in FIG. 9.

FIG. 11 the individual process stages for the production according to the invention of a tube fitting within a multistage press shown in simplified perspective representation, and

FIG. 12 a transverse cross-sectional view of a second embodiment of the split tool.

FIG. 11 shows the process stages I to VI for the production of a tube fitting 4 with outside thread according to the invention in simplified representation. The process stage relates to the cutting to length by means of a shearing tool 19 of a section 27 made of solid material (wire or bar-shaped material) fed from a supply roll. The subsequent process stages II to VI take place within a multistage press 20 which is constructed as a 5 stage press. The multistage press, which is not depicted in greater detail, has a machine frame of known construction per se with a displaceable carriage 21 as well as a stationary work piece carrier unit 22. The multistage press comprises a swaging tool 23 (process stage II), an initial reverse extrusion tool 24 (process stage III), a second reverse extrusion tool 25 (process stage IV), a punching tool 26 (process stage V) and a split tool 1, 8 as a pressing tool (process stage VI). To produce a copper tube fitting 4 with outside thread a blank 4 b with an annular collar is produced from the wire section 27 cut to length in the process stages II to V. The individual tools in these process stages II to V each comprise two dies, an upper die 28 a and a lower die 28 b, with a central open cavity to receive the blank 4 b, a punch 29 and an ejector 30 which is arranged in the opposite direction to the punch 29.

Die 28 a, is placed on one side of the tool (carriage or work carrier) and die 28 b on the other.

The extrusion punch 29 is introduced into the central cavity of the upper die 28 a and the ejector 30 or the ejector sleeve 7 is introduced into the opposite lower die 28 b. The movement of the punch 29 moves the carriage 21 of the multistage press. If necessary, the dies can also be constructed as a multipart split tool, depending on the geometry of the blank to be produced.

For punching, process stage V, a sleeve-like stop 7 has been provided instead of a punch. The first die is part of the carriage 21 or is mounted on this. The second die 28 b is part of the stationary work piece carrier unit 22. A reversed arrangement of the dies 28 a and 28 b would also be possible.

The prefabricated blanks 4 b are inserted into the tool of the subsequent process stage by means of gripper devices or handling devices on completion of a process stage. Similar arrangements also apply to the other stages. The shaping of the blank in the process stages II to IV preferably takes place as cold extrusion processes.

As shown in FIG. in 11 the copper wire or bar section 4 b, which has been fed in and cut to length, is reshaped into a tube fitting with an annular collar in the process stages II (upsetting), III (first dishing), IV (second dishing) and V (punching) mainly by upsetting and extrusion processes. Finally, an outside thread is to be produced on one of the two cylindrical sections of the tube fitting. This is carried out within the multistage press by a thread pressing process (process stage VI) by means of a multipart split tool 1 or 8. The respective split tool 1, 8 is fitted on the carriage 21 and the work piece carrier unit 22 in a similar way to which the tools of the other process stages are fitted, as shown in FIG. 11.

Special embodiments of the split tools for producing an outside thread are shown in FIGS. 1 to 6 and 12.

FIGS. 1 and 2 depict a two-part split tool 1 in longitudinal section to illustrate process stage VI in more detail. It comprises two pressing jaws of which only pressing jaw 2 is shown. The inner wall of the pressing jaw 2 that limits the cavity is fitted with a negative threaded profile 3. After the pressing jaws 2 have assumed the closing position, the prefabricated tube fitting or blank 4 b with an annular ring is inserted into the cavity 5. The rear section of the blank 4 b above the tubing collar has a larger internal diameter than the tubular section on which the thread is to be formed.

The tube fitting inserted is supported from below by an ejector sleeve 7 inserted into the cavity which serves as a stop to prevent a linear expansion of the tube fitting 4 b during the pressing process.

An expanding mandrel 6 with a bevelled tip is inserted from above into the central opening of the tube fitting 4 b to produce an outside thread on the tubular section of the tube fitting 4 b which projects into the cavity 5.

During the insertion of the expanding mandrel 6 a radial flow of material is produced in which the contours of the negative threaded profile 3 of the inner wall of the pressing jaws 1 are filled with material, leading to the formation of an outside thread on the tubular section of the tube fitting 4 b. The expanding process is concluded when the expanding mandrel 6 is inserted into the sleeve 7. The mandrel 6 then moves back to its starting position and the split tool 1 is opened. The completed tube fitting 4 with outside thread is then removed from the opened split tool 1 by means of the ejector sleeve.

The outer diameter of the rear section 6 a of the expanding mandrel 6 has been reduced in order to reduce the friction arising during the insertion.

FIGS. 3 to 6 depict a further embodiment variant as split tool 8 with three movable jaws 10, 11, 12 (FIG. 6). The split tool 8 comprises as a component of the lower die an external ring 9 displaceable in an axial direction in which three jaws 10, 11, 12 are mounted so as to be radially movable. The outer surfaces of the jaws 10, 11 and 12 have a conical surface area 17 and the external ring 9 has a conical inner surface 18 corresponding with this such that the jaws 10, 11 and 12 are movable into an opening and closing position within the external ring 9. The jaws 10, 11, 12 have a negative threaded profile 3 on their inner side or inner wall. The inner wall of the jaws limits the cavity 5 which is open on both sides up and below in the closed state of the jaws 10, 11, 12.

An annular component 13 (upper die) connected to the displaceable carriage of the multistage press sits on the external ring 9. The jaws 10, 11, 12 are opened and closed by the movement of a spring assembly that is not shown in any greater detail. The spring assembly is tensioned with the in-feed motion of the carriage by the axial movement of the annular component 13 which is in contact with the ring 9, wherein process ring 9 with the jaws 10, 11, 12 is moved against a stop 14. With movement in the opposite direction, the spring assembly is relaxed and the ring 9 is moved upwards in the process, with the jaws opening. The device also includes the expanding mandrel 6 and the sleeve-like ejector 7 which also serves as a stop. The axial movement of the expanding mandrel 6 is controlled by the movement of the carriage. FIG. 3 shows the split tool 8 in the closed tensioned condition. The blank or tube fitting 4 b has been transferred from the tool of the upstream process stage V by means of a gripping device which is not shown in detail, and is inserted in the cavity 5 of the split tool 8. The charging takes place with the tool open. The tube fitting 4 b is supported from underneath by an ejector sleeve 7 under spring tension.

The expanding mandrel 6, is then inserted from above into the central cavity of the work piece (tube fitting) as shown in FIG. 4 via the further in-feed motion of the carriage. An outside thread is produced on the tubular section of the tube fitting 4 b by the expanding process as previously explained.

On completion of the shaping process the expanding mandrel 6 and the annular component 13 are moved back to their starting positions. The spring assembly is relaxed, causing the pressing jaws 10, 11, 12 to be moved into the opening position (FIG. 5). The ejector sleeve 7 which is under initial tension is released in the opened condition of the split tool. This takes on the function as ejector for the formed part 4 which is now finished.

Instead of the mandrel 6 being moved the closed split tool can also be moved via the mandrel. This depends on the design layout of the multistage press.

In order to prevent the formation of a longitudinal edge in the area of the closing edges of the split tool during the expanding process, recesses or indentations 4 a can be formed running in a longitudinal direction on the tubular section of the blank on which the thread is to be produced. These must be arranged radially at the points where the closing edges of the pressing jaws 10, 11, 12 of the split tool are located, as shown in FIG. 6. The blank must then also be inserted correspondingly into the tool.

FIG. 12 shows another alternative for this in which the tool is depicted in the closed state and without blank or tube fitting. The negative threaded profile 3 of the individual jaws 10, 11, 12 is so formed that the depth of profile or depth of thread 3 a of the individual threads is reduced in each case in the direction of the closing gap or edge of the jaws 10, 11, 12. There is then no negative threaded profile in the immediate area of the closing edges of the jaws 10, 11, 12.

This considerably reduces the risk of material getting into the closing gap because of the extrusion process.

The two aforementioned measures can also be combined.

FIGS. 7 to 10 show further embodiment variants of expanding mandrels with different geometric shapes. The expanding mandrel 15 (FIGS. 7 and 8) has two different form shaping zones. The cross sectional shape of the upper section 15 a, which is inserted first into the cavity of the work piece 4 b, is formed as a triangle 15 b with rounded corners. During the insertion of this section into the cavity of the work piece 4 b, material flows initially in the sphere of action of these corners only into the contours of the threaded profile 3. The cross sectional shape of the subsequent section 15 c is adapted to the cavity, e.g. with a tubular cavity this has a circular cross sectional shape.

The expanding mandrel 16 shown in FIGS. 9 and 10 is used when the cavity of the work piece 4 b is divided into individual chambers running in a longitudinal direction. In its upper section 16 a, which is introduced into the divided cavity, the expanding mandrel 16 has two longitudinally running mandrel sections 16 b and 16 c between which a gap or free space 16 d is arranged. The material then flows radially into the contours of the threaded profile over the surface areas of the mandrel sections 16 b and 16 c. The outside thread so produced is interrupted in a longitudinal direction at the opposite points where the gap or free space 16 d is located. 

1-21. (canceled)
 22. A method of producing an outside thread on a hollow metal work piece, the method which comprises: providing a multistage press with a displaceable carriage and a stationary work piece carrier unit; feeding a base material of wire or bar stock within the multistage press and reshaping the base material in stages in one or more reshaping stages by extrusion and upsetting processes into a prefabricated hollow blank with a central opening; inserting the prefabricated hollow blank in a further stage within the multistage press into a multipart split tool, the split tool having a cavity with an inside wall formed with a negative threaded profile, having a plurality of jaws with closing edges; with the split tool closed, inserting at least one mandrel into the central opening of the blank and expanding the blank with the mandrel, thereby filling contours of the negative threaded profile by a radial material flow of the hollow blank and thereby forming the outside thread on the hollow blank; and preventing material of the hollow blank from entering into spaces between said jaws, when said jaws are forced apart during the expanding step, by one or both of the following: providing the negative threaded profile in a region of the closing edges of the jaws with a reduced profile depth relative to a remaining region thereof, and thereby producing a relatively reduced threaded profile in those regions of the blank that are located in a region of the closing edges of the jaws; or providing the hollow blank with small recesses in an area thereof intended for shaping the thread, the recesses running in a longitudinal direction of the hollow blank at locations where the jaws of the split tool meet; and subsequently opening the split tool and removing the hollow blank in the form of a finished work piece from the split tool.
 23. The method according to claim 22, which comprises securing the blank inserted into the split tool during a movement of the mandrel against elongation in a longitudinal direction.
 24. The method according to claim 22, which comprises controlling an opening movement and a closing movement of the jaws of the split tool, and a movement of the mandrel, by a central drive unit of the multistage press.
 25. The method according to claim 22, wherein the central opening of the hollow blank has a cross-sectional shape selected from the group consisting of a circular shape, an oval shape, and a trilobular shape.
 26. The method according to claim 22, wherein the cavity of the blank is divided into a plurality of cavity sections by at least one partition, and the method comprises introducing a mandrel with a plurality of mandrel sections into the cavity.
 27. The method according to claim 22, wherein the limiting wall surrounding the cavity of the blank is interrupted at one or more points.
 28. The method according to claim 22, which comprises providing the threaded profile with a plurality of thread sections having mutually different thread geometries, and producing thread sections of different thread geometries during the expanding step.
 30. The method according to claim 22, which comprises producing threaded profiles containing other profiles or producing other profiles outside the threaded profile.
 31. The method according to claim 22, which comprises providing a mandrel with different form shaping zones for shaping the outside thread, the mandrel having a first section with a polygonal contact surface with rounded corners and the inner wall of the blank being extended radially only in the sphere of action of the corners, and the mandrel subsequently having a second section matched to a cross sectional shape of the cavity of the work piece and assuming a final shaping of the thread during the expanding process.
 32. The method according to claim 22, wherein the hollow blank in the area intended for shaping the thread is formed with small recesses running in a longitudinal direction arranged at the locations where the jaws of the split tool meet.
 33. A device for carrying out the method according to claim 22, which comprises: a multistage press with a displaceable carriage and a stationary work piece carrier unit; at least one multipart split tool disposed within said multistage press either on said carriage or on said stationary work piece carrier unit, said multipart split tool having jaws and being movable into an opening position and a closing position and having a cavity limited by said jaws with at least one central opening into which a hollow blank can be inserted; wherein an inner wall of said jaws limiting said cavity is formed with a negative threaded profile, said negative threaded profile of each of said jaws having a depth of thread continuously decreasing on both sides towards a marginal edge thereof, so that a thread profile is produced in a transition area at the adjoining edges of said jaws that is substantially reduced or no thread profile is produced; at least one expandable mandrel configured for insertion into said hollow blank; and a drive unit for driving at least one of said mandrel or said multipart split tool.
 34. The device according to claim 33, which further comprises, in addition to said split tool, a further swaging tool, at least one reverse extrusion tool and a punching tool disposed within said multistage press, said tools being fitted with dies, with a die positioned on said carriage and the other die opposite positioned on said work piece carrier unit.
 35. The device according to claim 33, which comprises at least one stop disposed for securing a position of the blank during an expanding process.
 36. The device according to claim 33, wherein said cavity limited by said inner wall of said jaws is so dimensioned that there is only a limited play or contact between a surface area of the insertable blank and said inner wall of said cavity in the closed state of said jaws.
 37. The device according to claim 33, wherein the threaded profile of said jaws is interrupted by other profiles.
 38. The device according to claim 33, wherein said jaws outside the threaded profile are fitted with other profiles.
 39. The device according to claim 33, wherein said expandable mandrel has different form shaping sections, with a first section configured for insertion into said blank having a polygonal construction with corners or edges coming into contact with said blank being rounded.
 40. The device according to claim 34, wherein said expandable mandrel comprises a plurality of longitudinally running sections or segments, between which a free space is formed in a radial direction.
 41. In combination with a device for the non-cutting forming of an outside thread on a metal work piece, the device including: a multipart split tool having jaws being movable into an open position and a closed position in which a cavity is defined by said jaws with a central opening, said jaws defining an inner wall limiting the cavity formed with a negative threaded profile; at least one expandable mandrel and a drive unit for driving the mandrel and/or the multipart split tool; a metal work piece, comprising: a hollow blank formed for insertion into the central opening of the multipart split tool and having an interior formed for receiving therein the expandable mandrel; said hollow blank having an outside surface intended for shaping the thread formed with small recesses running in a longitudinal direction of said hollow blank at locations where the jaws of the split tool meet when said hollow blank is inserted into the central opening and the mandrel is being expanded. 